1. Field of the Invention
The present invention relates to an audio signal supply apparatus that supplies audio signals to a speaker array that forms a sound field, and more particularly, to an audio signal supply apparatus that makes it possible to broaden an area where satisfactory localization of sound images can be obtained.
2. Description of the Related Art
To reproduce realistic sound in acoustic spaces such as concert halls, movie theaters, and various event halls, a speaker array comprised of a plurality of speakers (hereinafter also referred to as “speaker units”) is commonly set up in the acoustic spaces. In an example illustrated in FIG. 6, nine listeners HM1 to HM9 are seated in three lines and three rows in an acoustic space RM which is rectangular in horizontal cross-section, and a speaker array 200 is placed on a front wall of the acoustic space RM. The speaker array 200 is comprised of, for example, a plurality of speakers which are two-dimensionally arranged. In the case where the speaker array 200 forms a sound field in which virtual sound images of right and left channels are localized on right and left side walls, respectively, of the acoustic space RM, delays and signal levels of audio signals which are to be supplied to the respective speakers of the speaker array 200 are adjusted as described below, for example.
Specifically, as shown in FIG. 6, it is assumed that there is a virtual image RM′ of the virtual space RM, which is symmetrical to the acoustic space RM with respect to the left side wall of the acoustic space RM. In FIG. 6, HM5′ denotes a virtual image of the listener HM5. A focus Ptg for left-channel audio signals is set at a point at which a straight line connecting the center of the speaker array 200 with the virtual image HM5′ of the listener HM5 intersects the left side wall. As a result, left-channel virtual sound images are formed at the focus Ptg. This also applies to right-channel audio signals.
However, if the focus Ptg is set in the above described way, there is the problem that the wave front of sound waves output from the speaker array 200 and reflected on the left side wall becomes wider as the distance from the focus Ptg increases, and hence, the more distant from the focus Ptg a listener is seated, the less is satisfactory the localization of virtual sound images he/she can obtain.
To solve this problem, a variety of techniques have been conventionally proposed. An example of such techniques is disclosed in U.S. Patent Publication No. 2004/0131338 A1. As shown in FIG. 7, this publication describes that the focus Ptg is set within the virtual image HM5′ so that an area where satisfactory localization of virtual sound images can be obtained (hereinafter referred to as “listening area”) is formed around the listener HM5.
Also, as shown in FIG. 8, to broaden the listening area, placing a diffusion plate (RB) on a wall surface defining the acoustic space RM so as to diffuse acoustic beams (sound waves) output from the speaker array 200 has been proposed.
According to the technique disclosed in U.S. Patent Publication No. 2004/0131338 A1, localization of sound images can be improved at locations in the directions of directivity axes of acoustic beams (in FIG. 7, the direction of a straight line connecting the listeners HM1, HM5, and HM9), but cannot be improved at locations out of the directions of directivity axes of acoustic beams, such as the locations of the listeners HM3 and the listener HM7. Namely, the technique disclosed in U.S. Patent Publication No. 2004/0131338 A1 has the problem that the listening area cannot be broadened satisfactorily. This problem does not arise if the technique involving architectural measures such as the placement of a diffusion plate is adopted, but there is the problem that efforts to take such architectural measures are needed.